Belt pulleys which are constructed from two separate pulley halves are known. It is a disadvantageous feature of the known belt pulleys that the concentric end surfaces through which the shaft passes are substantially flat so that the attachment of the pulley on the shaft requires the use of a, sometimes large, number of shims or spacer discs. Moreover, the area of contact between the inside of the hole of the pulley and the outside surface of the shaft is relatively small so that only relatively weak frictional forces come into play and the pulley must be held on the shaft almost exclusively due to forces exerted by the lock nut which exerts axial force on the hub of the pulley, thereby holding it in position. A particular disadvantage of this manner of fastening the pulley is that when the nut is tightened, the pulley may be rotated with respect to the shaft unless special devices are provided to prevent such relative rotation. Such rotation is undesirable because the belt pulleys are often required to maintain angular alignment with the shaft and may carry markings which must be placed in a particular relation with respect to other parts of the machinery.